1. Field of the Invention
This invention relates to a developing apparatus in the picture forming devices such as the copying machines, laser beam printers, and the printers to which magnetic latent image is applied or more particularly to a developing apparatus capable of developing (including magnet brush development, jumping developments, etc.) the electrostatic image on the electrostatic image holder (including the light sensitive bodies capable of holding electrostatic images).
2. Description of the Prior Art
Until now, in the developing apparatus of copying machines etc. (including other devices which have developing method and apparatus to form electromagnetic image and develop the image) the system which can be called to indiscriminate developing methods such as the fur brush developing method, cascade developing method, magnet brush developing method and powder cloud developing method in which the toner is made to contact with the whole area (the picture area and no-picture area which should not absorb the toner) on the electrostatic image holding body surface and the toner is left only to the picture area has been known.
In this system, however, the so called generation of fogging phenomenon in which some toner sticks to and remains on the no-picture area on the surface of the electrostatic image holding body cannot be avoided. To prevent this fogging phenomenon from being generated in magnet brush developing method the sticking of the toner is reduced by weakening the magnetic field at the developing section in order to reduce the amount of the toner sticking to the no-picture area. However, when this is done, the sticking of the toner to the picture area is also reduced, resulting in low picture density.
In the toner projection development system, one of the so-called jumping development systems described in U.S. patent application Ser. Nos. 58,434, now abandoned, and 58,435, now U.S. Pat. No. 4,292,387 by this assignee in which the surface of the electrostatic image holding body and the developing roller etc. are arranged face to face and development is made by letting the toner travel back and forth between them, although the fogging is surely reduced compared with the conventional system complete avoidance of the generation of fogging phenomenon was a difficult matter. In the works, although it is considered to strengthen the binding of the toner to the developing roller by intensifying the magnetic field of the developing pole of the developing section to reduce the jumping of the toner to the no-picture area, in this use also the reduction of picture density cannot be avoided since the jumping of the toner to the picture area is reduced with the reduction in the toner jumping to the no-picture and similar to the above-mentioned case.